The easiest thing to do would be to put a BIOS-password on it. That way, all you can do if you turn the machine on, is guessing the password. And that's not really fun. This implies that only you and the people you tell that password can use the machine.
There are other possibilities, but most can be fooled by any user who knows how you set it up. So if you're happy with the above, that would be the way to go. If not, post again. Then I'll tell you how to edit autoexec.bat to append a line with date and time of each boot to a 'log-file' you can examine in Notepad. But, as I said, it's easy to forge for anybody who's more computer litterate than you are. Other people might come with more sophisticated ideas.
Hope this helps.
I am mostly computer illterate so if this seems silly I apologize, but can you tell if your computer has been turned on without your knowledge? Do computers keep track of that and if so how can I find that out?